If space in the universe begins to contract, subsequent events in the timeline may not occur because the Big Crunch, the collapse of the universe into a hot, dense state similar to that after the Big Bang, will supervene. This future history and the timeline below assume the continued expansion of the universe. Finally, in the Dark Era, even black holes have disappeared, leaving only a dilute gas of photons and leptons. In the Black Hole Era, white dwarfs, neutron stars, and other smaller astronomical objects have been destroyed by proton decay, leaving only black holes. In the subsequent Degenerate Era, the stars will have burnt out, leaving all stellar-mass objects as stellar remnants- white dwarfs, neutron stars, and black holes. It is the time during which stars form from collapsing clouds of gas. The second, the Stelliferous Era, includes the present day and all of the stars and galaxies now seen. The first, the Primordial Era, is the time in the past just after the Big Bang when stars had not yet formed. Then, in their 1999 book The Five Ages of the Universe, the astrophysicists Fred Adams and Gregory Laughlin divided the past and future history of an expanding universe into five eras. In the 1970s, the future of an expanding universe was studied by the astrophysicist Jamal Islam and the physicist Freeman Dyson. It is possible that the dark energy equation of state could change again resulting in an event that would have consequences which are extremely difficult to parametrize or predict. If the theory of inflation is true, the universe went through an episode dominated by a different form of dark energy in the first moments of the Big Bang but inflation ended, indicating an equation of state much more complicated than those assumed so far for present-day dark energy. If, as in the concordance model of physical cosmology (Lambda-cold dark matter or ΛCDM), dark energy is in the form of a cosmological constant, the expansion will eventually become exponential, with the size of the universe doubling at a constant rate. The acceleration of the universe's expansion has also been confirmed by observations of distant supernovae. In this case, the universe might continue to expand at an accelerating rate. Observations of the cosmic background radiation by the Wilkinson Microwave Anisotropy Probe and the Planck mission suggest that the universe is spatially flat and has a significant amount of dark energy. It can be open (with negative spatial curvature), flat, or closed (positive spatial curvature), although if it is closed, sufficient dark energy must be present to counteract the gravitational forces or else the universe will end in a Big Crunch. Infinite expansion does not determine the overall spatial curvature of the universe. Ultimately, if the universe reaches thermodynamic equilibrium, a state in which the temperature approaches a uniform value, no further work will be possible, resulting in a final heat death of the universe. According to theories that predict proton decay, the stellar remnants left behind will disappear, leaving behind only black holes, which themselves eventually disappear as they emit Hawking radiation. ![]() As existing stars run out of fuel and cease to shine, the universe will slowly and inexorably grow darker. Stars are expected to form normally for 10 12 to 10 14 (1–100 trillion) years, but eventually the supply of gas needed for star formation will be exhausted. Redshift will stretch ancient, incoming photons (even gamma rays) to undetectably long wavelengths and low energies. ![]() If dark energy-represented by the cosmological constant, a constant energy density filling space homogeneously, or scalar fields, such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space-accelerates the expansion of the universe, then the space between clusters of galaxies will grow at an increasing rate. For this reason, this future scenario once popularly called " Heat Death" is now known as the "Big Chill" or "Big Freeze". The prevailing theory is that the universe will cool as it expands, eventually becoming too cold to sustain life. Observations suggest that the expansion of the universe will continue forever.
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